Color filters of a liquid crystal display device and the like each have a red pixel portion (R), a green pixel portion (G), and a blue pixel portion (B). Any one of these pixel portions has a structure in which a synthetic resin thin film containing an organic pigment dispersed therein is provided on a substrate, and an organic pigment of each of the red, green, and blue colors is used as the organic pigment.
As a blue organic pigment used for forming the blue pixel portion among these pixel portions, an ε-type copper phthalocyanine pigment (C. I. Pigment Blue 15:6) is generally used, and if required, it is combined with a small amount of a dioxazine violet pigment (C. I. Pigment Violet 23) as a violet organic pigment for toning.
The organic pigments used for forming a color filter are required to have characteristics completely different from those for conventional general applications, specifically, the characteristic of more brightening (increasing brightness) a display screen of a liquid crystal display device or the like. However, high brightness cannot be achieved by combination of the ε-type copper phthalocyanine pigment and the dioxazine violet pigment, and thus particularly, the organic pigment used in the blue pixel portion (B) is required to increase brightness.
In order to comply with such higher brightness, using a triarylmethane pigment, such as C. I. Pigment Blue 1, which is superior in brightness to the ε-type copper phthalocyanine pigment, in a blue pixel portion of a color filter has been recently being investigated (Patent Literatures 1 to 3).
Fanal Color (Fanal Blue D6340 and 6390) of BASF Corporation having a chemical structure below is notable as the C. I. Pigment Blue 1, and the C. I. Pigment Blue 1 is produced by laking Victoria Pure Blue BO, which is a basic triarylmethane dye, with a heteropoly acid such as phosphomolybdic acid, phosphotungstomolybdic acid, or the like. The resultant C. I. Pigment Blue 1 includes counter ion X− represented by Keggin-type PMoxW12-xO40 for a cation.
[In general formula (II), R1, R2, and R3 are each an ethyl group, R4 is a hydrogen atom, and X− is a Keggin-type phosphotungstomolybdic acid anion or phosphomolybdic acid anion.]
However, even by using the triarylmethane pigment for preparing the blue pixel portion of a color filter, satisfactory brightness cannot yet be maintained over a long period of time under a high temperature of 200° C. or more or under light irradiation, and the actual condition is that heat resistance and light resistance are unsatisfactory.